1. Field of the Invention
The present invention relates generally to dynamic dampers as one type of vibration damping devices installed on vibrative members in order to attenuate vibration excited in the vibrative members. More specifically, the present invention is concerned with a dynamic damper of novel structure that is capable of exhibiting desired damping effect more precisely and stably than conventional dynamic dampers, and is preferably usable as a dynamic damper for a steering wheel of an automotive vehicle.
2. Description of the Related Art
A steering wheel of an automotive vehicle is a member that is almost always directly handled by an operator during driving of the vehicle, thus the operator of the vehicle is able to sensitively feel minute vibration excited in the steering wheel. Namely, the vibration excited in the steering wheel noticeably affect a riding comfort of the vehicle as felt by the operator, whereby a great importance has been attached on absorbing vibration excited in the steering wheel, in the field of vibration damping for automotive vehicles. Structurally, the steering wheel is attached to a distal end of a longitudinal steering shaft (or steering column) that is fixedly supported at its proximal end, while adding a somewhat mass to the distal end of the steering shaft. For this structural feature, the steering shaft is likely to undergo swinging displacement, thus exhibiting vibration in the steering wheel in some directions orthogonal to an axis of the steering shaft, e.g., vertical and lateral directions of the vehicle. Furthermore, a natural frequency of a steering system including the steering shaft and the steering wheel is usually held within a range of about 20-40 Hz, thereby causing a significant problem of vibration in the steering wheel.
To cope with this problem, it has been proposed to mount a dynamic damper of specific structure on the steering wheel or any other member of the steering system so as to attenuate vibration in a frequency band of idling vibration. Examples of such a dynamic damper are disclosed in Patent Document 1 (JP-A-10-267075) and Patent Document 2 (JP-B-2824382).
The dynamic damper disclosed in Patent Document 1 or 2 includes: a metallic mass member; a metallic support plate fixable to a vibrative member; and an elastic connecting member formed of a rubber elastic body or the like and elastically connecting the mass member and the support plate such that the mass member is effectively and stably displaceable relative to the support plate in a plurality of vibration input directions orthogonal to the axis of the steering shaft, e.g., generally vertical and lateral directions of the vehicle. Therefore, the disclosed vibration damper is able to exhibit damping effect with respect to vibration applied thereto in the plurality of vibration input directions.
As well known in the art, the mass member and the elastic connecting member of the dynamic damper cooperate to provide a mass-spring system functioning as a secondary vibration system with respect to a primary vibration system in the form of the vibrative member. A natural frequency of this secondary vibration system is precisely tuned to a frequency of vibration to be damped in the vibrative member, i.e., in the primary vibration system, whereby the vibration damper can exhibit intended damping effect. Therefore, significant for the dynamic damper are: (i) a highly precise tuning of the natural frequency of the secondary vibration system; and (ii) a long-term stability and sustention in tuning of the natural frequency of the secondary vibration system.
In this regards, the dynamic damper of conventional structure disclosed in the above indicated Patent Document 1 or 2 merely includes a support portion for supporting the mass member via the elastic connecting member, which support portion is formed by simply bending longitudinally opposite side edges of a metallic support plate of thin flat-plate shape. An extensive study by the inventors on the conventional dynamic dampers has revealed that the support portion of above-described structure is insufficient in its strength and rigidity, and is prone to cause variation in shape or dimension from the preset ones at time of manufacture or in the course of practical use. It is not difficult to expect the occurrence of somewhat deformation of the support portion, since a load due to the displacement of the mass member is directly exerted on the support portion via the elastic connecting member. In addition, since the support member is configured to sandwich the mass member in one direction via the elastic connecting member, the change in shape or dimension of the support member directly leads to a change in spring characteristics of the elastic connecting member. As a result, the preset tuning of the dynamic damper is undesirably changed, failing in exhibiting intended damping effect with stability.
The extensive study by the present inventor on the disclosed dynamic damper of conventional structure has also confirmed that when the elastic connecting member is formed of a rubber elastic body, it is effective to pre-compress the rubber elastic body formed by vulcanization of a rubber material, by deforming the support portion after the vulcanization of the rubber material is executed. Namely, it is useful for minimizing a tensile stress in the rubber elastic body due to its contraction during the vulcanization, thus improving durability of the dynamic damper. However, the support portion of the dynamic damper of conventional structure is formed by simply bending the side edges of the support plate of thin-flat-plate shape. If the support portion of the conventional dynamic damper is deformed in order to pre-compress the elastic connecting member, the support portion undergoes a relatively large amount of spring back owing to its structural feature, and it is extremely difficult to control the amount of spring back of the support portion. This makes it further difficult to ensure precision in tuning of the dynamic damper.